1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using a conductive polymer as a solid electrolyte layer.
2. Description of the Background Art
In connection with size and weight reductions of electric equipment in recent years, a small capacitor of a large capacity for high frequencies, which has a low impedance in a high frequency region, has come to be required.
Though a mica capacitor, a film capacitor, a ceramic capacitor, and the like are used as capacitors for high frequencies, these capacitors are not suitable for large capacities.
On the other hand, an aluminum electrolytic capacitor, a tantalum electrolytic capacitor and the like are suitable for large capacities. Though a large capacity can be attained at low cost with the aluminum electrolytic capacitor, there are problems such as a change of electrolyte solution used over time because of evaporation and a high impedance at a high frequency.
Since the tantalum solid electrolytic capacitor uses solid manganese dioxide as an electrolyte, degradation in capacity of the capacitor is small. The solid electrolyte of the tantalum solid electrolytic capacitor, however, is formed by impregnation of sintered tantalum with aqueous solution of manganese nitrate followed by pyrolysis of manganese nitrate at about 350° C., which impregnation and pyrolysis steps usually have to be repeated for several to tens times. Therefore, considerable labor is required in formation process of the solid electrolyte. In addition, since a coating of manganese dioxide is deficient in self-repairing, there is a possibility of ignition or the like when a dielectric coating is damaged while a current is passed therethrough.
In order to solve problems mentioned above, use of a conductive polymer, which has a good electrical conductivity and is easily formed to be a solid electrolyte, as a solid electrolyte has been proposed recently (see Japanese Patent Laying-Open No. 60-037114 and No: 60-244017). With this technique, a solid electrolytic capacitor can be obtained which, in comparison with the solid electrolytic capacitor as described above, requires lower manufacturing cost, ensures a capacitance, has a dielectric coating that is not damaged, and has small leakage current.
The conductive polymer is formed by electrolytic polymerization of heterocyclic monomers such as pyrrole, thiophene and furan with a supporting electrolyte to form a film of polymer having a good conductivity on the dielectric coating using anion of the supporting electrolyte as a dopant. As the dopant of the conductive polymer, a halide such as a perchlorate ion or a boron tetrafluoride ion, a para-toluenesulfonate ion, a dodecylbenzenesulfonate ion, or the like is used.
For increasing reliability of such solid electrolytic capacitor, a decrease in an ESR (Equivalent Series Resistance) and an increase in a heat resistance are needed. A solid electrolytic capacitor using tetrahydronaphthalenesulfonic acid as a dopant of a conductive polymer has been proposed recently (see Japanese Patent Laying-Open No. 2003-158043).
Though tetrahydronaphthalenesulfonic acid is highly effective in decreasing the ESR, as it has low heat resistance, the solid electrolytic capacitor using tetrahydronaphthalenesulfonic acid as a dopant has the ESR which is low before a reflow but is increased after the reflow.